The invention relates to a three-level converter. A three-level converter is a circuit that uses power semiconductors (semiconductor switches and/or diodes, in particular) to convert a supplied input voltage into an output voltage wherein, in the static case in particular, this output voltage can assume three discrete values (or potential levels), for example the values “0”, “½” and “1” in relation to the input voltage at any instant in time.
The three-level converter is in particular a three-level inverter, which converts a DC voltage supplied on the input side into an AC voltage output to a load circuit. The invention relates in particular to a three-level converter that is designed for use in the Megawatt range (e.g. in the range of 1 MW-20 MW), for example for a wind turbine or in a rolling mill.
In a three-level converter of this type, single faults (i.e. a failure of a single power semiconductor in the converter) can result in the failure of further power semiconductors as consequential faults. In particular, a failure of the power semiconductor concerned to go into the off-state causes the power semiconductor to be permanently on (even if it happens to be installed in a reverse-bias direction). The failure of a plurality of power semiconductors can result in low-inductance short-circuit paths within the converter or to multiphase short-circuits for the connected load. The short-circuit current that occurs in this case can cause a high transient torque in an electrical machine connected as a load to the converter. The transient torque anticipated for the short-circuit situation may be relevant to the design of the mechanical system of the connected electrical machine, and hence may require the mechanical design of this machine to be over-engineered.
In addition, the transient current in the event of a short-circuit results in a considerable load on the diodes in the load-side short-circuit path, and in the case of a complete bridge short-circuit even on all the diodes connected to the same DC circuit. In order to protect the diodes from damage by the transient current, chokes are often connected in series with a converter on the AC side. These chokes significantly increase costs, mass and occupied space. Furthermore, these chokes also cause higher losses and a lower output voltage.